Screw and rod fixation system

ABSTRACT

A pedicle screw and rod fixation assembly including a pair of opposing tabs having a proximal end and a distal end, the pair of opposing tabs being coupled to one another about the distal end thereof by a screw head member, a decoupling mechanism for decoupling the pair of opposing tabs from the screw head member, a rod receiving slot between the pair of opposing tabs, and a sliding member configured for coupling to the pair of opposing tabs. The sliding member and the pair of opposing tabs are configured for allowing movement of the sliding member distally along the pair of opposing tabs and preventing movement of the sliding member proximally along the pair of opposing tabs.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/080,482, filed on Apr. 5, 2011 and titled, “Screw and Rod FixationSystem,” the entire contents of which are incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to spinal implants. More particularly, thepresent invention relates to screw fixation assemblies for use with bonefixation systems.

BACKGROUND OF THE INVENTION

Spinal deformity surgery is a complex technical procedure that caninvolve numerous implants and techniques to achieve a straightening ofthe spine in three dimensions. There is a sagital plane that may containan increased or decreased lordosis or kyphosis, a coronal plane that maycontain a left/right curvature of the spine and an axial plane that maycontain a rotational deformity. All three planes must be addressed if aresult that leaves the patient functional and provides acceptablecosmesis is to be achieved.

Current techniques involve the use of pedicle screws and various hooksas anchors or fixation points along the posterior spine. Screw types caninclude polyaxial screws, fixed angle or monoaxial screws and uniplanarscrews. Each screw type has a feature set that can be useful in varioussituations. As such, surgeons will select a screw type for a particularprocedure based upon the problems to be addressed and the feature set ofthe screw type.

Following placement of the fixation points by a physician, thecorrective part of the procedure occurs when a rod is introduced intoeach of the fixation points along the spine. Except in extreme cases, asingle “correcting” rod is placed first on one lateral side of thespine, followed by second “stabilizing” rod on the other lateral side ofthe spine. The rod is contoured to a shape that closely approximates thenatural contour of a non-deformed spine. After the rods are introducedinto the fixation points, the physician forces the fixation point tomeet the rod so that the rod can be locked into the fixation point,which may be a screw or a hook, to hold the spine in a more normalposition.

In certain cases, the physician is unable to introduce the rod into allof the fixation points or bring the rod and fixation points together.This can occur if the spine is severely deformed which causes thefixation points to be misaligned. When this occurs, the physician bendsthe rod in such a way that it can be introduced into the fixationpoints, but this compromises the correction.

Bending of the rod “in situ” can be used to provide more correctionafter the rod is seated into all of the fixation points. Obviously, theintroduction of a rod into a deformed spine can be very challenging todo so there have been many instruments designed to help forcibly bringthe spine and the rod together. Often, when an instrument is used toforce the mating of the rod and anchor, the anchor can pull out of thebone because the force is too great. Also, while reducing the rod intoone anchor, it may miss the anchor above or below because the alignmentof the rod and the spine do not match. In this case, the rod must bereleased from the anchor and re-introduced taking care, and oftenanother set of hands, to guide the rod into the adjacent anchors duringreduction.

Reduction screws have been used widely for some time to aide in theintroduction of a rod into a fixation point and forcing of the rod intoa seat of the screw. Reduction screws have extended arms or tabs thatare threaded so that a set screw can be introduced into the extendedtabs for securing the rod into the screw before the rod is fully seatedin the screw seat. By turning the set screw, the rod is driven down theextension tabs and into the screw seat of the screw head in a controlledand progressive manner.

A shortcoming of current reduction screws is that a physician utilizinga reduction screw must turn the set screw ten to fifteen complete turnsto seat the rod. Since it is not uncommon for twenty or more screws tobe used during a corrective spine surgery, reduction screws areunderused as physicians are reluctant to invest the time and effortrequired to turn the set screws three hundred times. Another shortcomingof reduction screws is that their extension tabs rarely extend longerthan approximately one inch. This is in part due to the additional setscrew advancement, i.e., complete set screw turns, required when longerextension tabs are used. Further, longer extension tabs are prone to“splay” when reduction forces are applied. This can cause the set screwto disengage or cross-thread. Instruments have been designed to controlthe splaying effect, but they tend to complicate the procedure. Yetanother shortcoming of reduction screws is that the amount of torquethey require to drive a rod into the screw seat is difficult totranslate into actual downward force by the physician. A few inch poundsof torque created by rotating a screwdriver handle can equate to overone hundred pounds of force on the screw. This can cause the pediclescrew to pull out of the bone thus causing the loss of a fixation point.

SUMMARY OF THE INVENTION

The screw and rod fixation system of the present invention improves theprocedure for correcting defective spines by providing enhancedreduction capabilities without adding the need to drive a set screw theentire length of every extended tab. The reduction mechanism can be mademuch longer which facilitates rod introduction along the entire lengthof the spine prior to any forces being applied. Splaying of the extendedtabs is not an issue because a collar prevents the tabs from splayingoutward and the rod prevents inward splay. This also allows for thesimultaneous introduction of two separate rods which reduces the amountof localized force on any given anchor and distributes the correctiveload among many of the anchors.

More particularly, each pedicle screw is anchored in a vertebra in thestandard fashion. The extended tabs can be integrated into the screwwhen it is manufactured, or the extended tabs can be provided as atemporary extension that is placed onto the screw during surgery andremoved when a rod is fully seated. The integral extended tabs arebroken off at the end of the surgery as is typical of existing reductionscrews. Where two adjacent rods are being implanted, both rods areintroduced into the slots formed by the extended tabs for each screw asa first step. Collars are then placed onto the extended tabs to capturethe rods into the extended tabs. At this point, both rods are in placewith little or no force being applied to the spine.

Reduction of the rods into the bottom of the screw heads is carried outby pushing the collars down toward the base of the screw heads by handor with an instrument designed for this purpose. As the rods begin toencounter resistance, the spine begins to move in small incrementstoward the rods. As the collars are pushed farther down, a ratchetingaction between the collars and the extended tabs retains the rods inplace and prevents them from sliding back up the extended tabs. Thisallows the physician to sequentially move up and down the spine fromscrew to screw and on both sides of the spine to push the collars downon the extended tabs with an appropriate amount of force based on thenumber of anchors, bone quality, anchor quality etc.

Since this initial reduction is by hand the force applied is 1:1 suchthat the exact downward force applied by the surgeon is the same forceretained by the collar when the surgeon releases pressure. This offersimproved tactile feedback in a reduction maneuver over the prior art.The present invention also allows for distribution of the correctiveforces over the entire spine thereby reducing the chances of screwpullout.

As manual reduction continues, instruments that apply more force thancan be generated manually can be used to progressively force the collarsfarther down the extended tabs in a controlled fashion. To continue todistribute the forces, multiple instruments can be used simultaneouslysuch that a single anchor does not receive excessive force.Alternatively, as increased reduction force is applied with aninstrument, an assistant can manually advance the collars on the anchorsadjacent to the persuader instrument to immediately disperse the forceto multiple anchors. Alternatively, a physician can choose to initiallyplace a single rod with this technique and subsequently place the secondrod. The benefits are the same except that the corrective forces are notshared by the second rod, therefore increasing the load on the anchorsof the first rod placed.

When the rod reaches its final position in the screw heads, set screwsare introduced to hold the achieved correction. The tabs and collars arethen removed. Rod rotation and in situ bending can now be employed toachieve adjustments to the final correction, as with standard implants.

According to one aspect of the present invention, there is provided ascrew and rod fixation assembly including a pair of opposing tabs havinga proximal end and a distal end, the pair of opposing tabs being coupledto one another about the distal end thereof by a screw head member. Theassembly further includes a decoupling mechanism for decoupling the pairof opposing tabs from the screw head member, a rod receiving slotbetween the pair of opposing tabs, and a sliding member configured forcoupling to the pair of opposing tabs. The sliding member and the pairof opposing tabs are configured for allowing movement of the slidingmember distally along the pair of opposing tabs and preventing movementof the sliding member proximally along the pair of opposing tabs.

According to another aspect of the invention, there is provided anassembly including extension tabs detachably coupled to a pedicle screwhead, a rod receiving slot formed between the extension tabs, a collar,and a ratchet mechanism configured for preventing movement in apredetermined direction between the collar and the extension tabs. Theratchet mechanism is coupled to and between the collar and the extensiontabs and includes at least one pawl that is biased toward a central axisof the collar. In use, a rod is inserted into the rod receiving slot,the extension tabs are inserted through an opening in the collar, thecollar is slid along the extension tabs toward the screw head, and theratchet mechanism is used to prevent movement of the collar away fromthe screw head.

According to yet another aspect of the invention, there is provided amethod for reduction of a rod into a screw having detachable extensiontabs. The method includes inserting the rod within a slot formed betweenthe extension tabs, coupling a sliding member to the extension tabs,pressing the sliding member against the rod and sliding the slidingmember and rod together toward the screw. When the rod exerts a forceagainst the sliding member, the sliding member is prevented from slidingalong the extension tabs away from the screw by a ratchet mechanism.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a screw and rod fixation assembly inaccordance with a preferred embodiment of the present invention.

FIG. 2 is an exploded perspective view of the screw and rod fixationassembly of FIG. 1 displaying a set screw.

FIG. 3 is a top plan view of the screw and rod fixation assembly of FIG.1

FIG. 4 is a sectional view of a ratchet collar of the screw and rodfixation assembly of FIG. 1

FIG. 5 is a top plan view of the collar of FIG. 5.

FIG. 6 is sectional view of a ratchet collar pawl of the ratchet collarof the screw and rod fixation assembly of FIG. 1 and correspondingratchet grooves.

FIG. 7 is a sectional view of an alternative ratchet collar.

FIG. 8A is sectional view of yet another ratchet collar according to apreferred embodiment of the present invention.

FIG. 8B is a perspective view of the collar of FIG. 8A.

FIG. 9 is a sectional view of the screw and rod fixation assembly ofFIG. 1 illustrating placement of the rod into a slot defined by the pairof extension tabs.

FIG. 10 is a sectional view of the screw and rod fixation assembly ofFIG. 1 illustrating coupling of the ratchet collar to the pair ofextension tabs.

FIG. 11 is a sectional view of the screw and rod fixation assembly ofFIG. 1 illustrating use of the ratchet collar to press the rod into arod seat of the screw.

FIG. 12 is a sectional view of the screw and rod fixation assembly ofFIG. 1 illustrating placement of a set screw in between the pair ofextension tabs.

FIG. 13 is a sectional view of the screw and rod fixation assembly ofFIG. 1 illustrating removal of the pair of extension tabs and fixing ofthe rod into a rod seat of the screw.

DETAILED DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENTS

FIGS. 1 through 13 illustrate a screw and rod fixation assembly 10 andvarious parts thereof in accordance with a preferred embodiment of thepresent invention. Generally, assembly 10 includes a distal end 11, apedicle screw 12 having a screw head 14, a proximal end 13, a pair ofopposing extension tabs 16 detachably coupled to screw head 14, a rodreceiving slot 18 defined between extension tabs 16, a rod 20 slideablyreceived within slot 18, a ratchet collar 22 pressed against rod 20 andcoupled to tabs 16 in a manner allowing movement of collar 22 distallyalong tabs 16 while preventing movement thereof proximally and a setscrew 24 for locking rod 20 within screw head 14.

More particularly, referring to FIGS. 1 and 2, screw 12 can be any size,shape or design as is known to those of skill in the art for anchoringto bone. In the preferred embodiment, screw 12 is a monoaxial pediclescrew and is therefore integrally formed with screw head 14 which isaxially aligned with a longitudinal axis of screw 12. In thisembodiment, relative movement between screw 12 and screw head 14 is notallowed. In an alternative embodiment, screw 12 may be a polyaxial screwwhere screw head 14 is pivotably mounted to an upper end 26 of screw 12.In this embodiment, screw head 14 is allowed to pivot about screw 12which can reduce stress on the spinal column during rod reduction.

Extending proximally from opposing sides of screw head 14 are a pair ofopposing tabs 28. Tabs 28 form there between, along the inner surfacesthereof, a U-shaped rod seat 30 having proximally extending, opposing,arcuate-shaped walls for receiving rod 20 from rod receiving slot 18 andguiding rod 20 into screw head 14. The distance between the proximallyextending walls is substantially the same as the diameter of rod 20 sothat rod 20 can be snuggly received within rod seat 30. To maintain orlock rod 20 within rod seat 30, the proximally extending walls of rodseat 30 include threaded portions 32 that are arranged there along forreceiving set screw 24. Threaded portions 32 do not extend proximallyalong extension tabs 16 and are configured so that locking rod 20 withinrod seat 30 requires fewer turns of set screw 24 than current pediclescrews.

Detachably coupled to the proximal ends of tabs 28 are extension tabs16. Tabs 28 are separated from extension tabs 16 by notches 34 whichenable tabs 16 to be readily removed from tabs 28. Like tabs 28,extension tabs 16 have proximally extending, opposing, arcuate-shapedwalls for receiving rod 20 and guiding rod 20 into screw head 14.However, unlike tabs 28, the proximally extending walls of extensiontabs 16 have smooth inner surfaces 36, which together define therebetween slot 18. Further, the proximally extending walls of extensiontabs 16 are separated by a greater distance than the proximallyextending walls of tabs 28. These differences between tabs 28 andextension tabs 16, when taken together, allow the passing of set screw24 distally between extension tabs 16 without having to turn set screw24 while requiring set screw 24 to be turned when progressing set screw24 along threaded portions 32 of tabs 28.

Extending along the entire length of the exterior surface of each ofextension tabs 16 is a column of horizontally arranged grooves 38.Grooves 38 are divided from one another by a plurality of ridges.Together with grooves 38, the ridges are configured to interact withcollar 22 in a ratcheting manner to allow collar 22 to readily slidedistally along and between extension tabs 16 while preventing movementthereof proximally. In the preferred embodiment, the ridges betweengrooves 38 resemble pointed teeth having a distally angled proximal orupper side and a distal or lower side that is essentially perpendicularto a longitudinal axis of screw 12. However, any groove and/or ridgeconfiguration that is capable of allowing linear movement of collar 22or other suitable ratchet device distally along extension tabs 16 whilepreventing movement in the opposite direction will suffice.

Referring to FIGS. 3 through 5, collar 22 is composed of a rigid,cylindrical body having a central opening 40 therethrough. Opening 40has a diameter that is essentially the same as the outer diameter ofopposing extension tabs 16 as defined by the ridges on the outersurfaces thereof. Opening 40 is configured for receiving and slidingalong opposing extension tabs 16. To prevent the sliding of collar 22proximally along extension tabs 16, a pair of rigid, elongate, biasedpawls 42 are provided on opposing sides of collar 22. Each of pawls 42has a pivot point 44 coupled to collar 22, an exterior end 48 which isbiased against and protrudes outwardly from an exterior surface 50 ofcollar 22 by a spring 46, and an interior end 52 that protrudes radiallyinwardly through an inner surface 54 of collar 22 into opening 40.Interior ends 52 are configured for selectively mating with grooves 38of extension tabs 16 when collar 22 is coupled to the extension tabs. Inparticular, when collar 22 is coupled to extension tabs 16 and downwardpressure is applied to collar 22, interior ends 52 are caused to pivotand ride along the ridges and grooves 38 of extension tabs 16 therebyallowing distal movement of collar 22 along extension tabs 16. However,when upward pressure is placed upon collar 22, interior ends 52 arecaught on the ridges and within grooves 38 and no pivoting of pawls 42occurs by virtue of the geometries of the ridges, grooves 38 andinterior ends 52. Consequently, proximal movement of collar is arrested.In the event proximal movement is desired, springs 46 and exterior ends48 of pawls 42 can be depressed thereby disengaging interiors ends 52from grooves 38.

Extending from a lower edge of collar 22 are a pair of opposing, roundedprotuberances 56 that are arranged below and between opposing pawls 42.Protuberances 56 serve both to properly align collar 22 with extensiontabs 16 so that pawls 42 directly engage grooves 38 and to engage andpress rod 20 distally into rod seat 30. In particular, when placed ontoextension tabs 16, collar 22 is oriented so that protuberances 56 extendbetween the opposing extension rather along the exteriors surfaces ofthe tabs.

Referring to FIGS. 7 and 8, there are depicted two alternatives tocollar 22. In particular, FIG. 7 illustrates a cylindrical collar 22′having rigid body and a pair of flexible, elongate opposing pawls 58each having a lower end 60 integrally formed and extending upwardly froma lower edge 62 of collar 22′ and an upper end 61 arranged to protruderadially inwardly through an inner surface 66 of collar 22′ and engagegrooves 38 of extension tabs 16. Upper end 61 includes a pair of teeth,each tooth having a proximal side that extends substantiallyperpendicular to a longitudinal axis of screw 12 and a distal or lowerside that extends proximally and radially inwardly. Pawls 58 areconfigured to bend about lower ends 60 to allow the teeth of upper ends61 to pivot radially outwardly and ride along the ridges and groves 38of extension tabs 16 when collar 22′ is slid distally while preventingproximal movement. Each of pawls 58 is received within a slot 64 formedwithin inner surface 66 of collar 22′ for receiving pawls 64 when pawls64 are pressed radially outwardly by their interaction with the ridgesand grooves 38 of extension tabs 16.

FIGS. 8A and 8B illustrate a cylindrical collar 22″ having rigid bodycomposed of a left half 66 and a right half 68 that are pressed togetherby a pair of elastic rings 70 circumscribing collar 22″. A pair ofgrooves 72 encircling and formed within an exterior surface of left 66and right halves 68 is provided for receiving rings 70 and maintainingrings 70 in place about collar 22″. Extending inwardly from an innersurface 71 of collar 22″ are a plurality of fixed pawls 74 that consistof a pair of teeth like those described for collar 22′. However, unlikepawls 58 of collar 22′, pawls 74 are not flexible and are arranged toremain stationary relative to the respective left 66 and right 68 halvesfrom which they extend when engaged with grooves 38. Thus, pawls 74 arebiased inwardly by the action of rings 70 which are stretched uponengagement of collar 22″ with extension tabs 16, and more particularlyupon the riding of pawls 74 over the ridges formed by grooves 38. Rings70 bias pawls 74 against grooves 38 thereby enabling pawls 74 andgrooves 38 to provide the desired ratcheting effect described above forcollars 22 and 22′. As shown in FIG. 8A, pawls 74 can be placed about alower edge 76 of collar 22″ or adjacent to an upper edge 78 of collar22″ as depicted in FIG. 8B.

FIGS. 9 through 13 illustrate a method for reduction of rod 20 into aplurality of screw heads 14 using collar 22 and set screws 24. Referringto FIG. 9, following anchoring of a number of screws 12 to respectivevertebrae, rod 20 is positioned about a proximal end 13 of each of thecorresponding assemblies 10 and lowered toward a distal end 11 withinrod receiving slot 18 of each such assembly 10. Because of abnormalitiesin the alignment of the vertebrae, rod 20, which as used herein means asubstantially cylindrical body having a desired length, is preventedfrom readily sliding completely distally within slot 18 of each assembly10 and seating in rod seats 30.

To aid in the reduction of rod 20 into rod seat 30 of each assembly,collars 22 are placed over the proximal ends 13 and around extensiontabs 16 of each of assemblies 10. Each collar 22 is arranged so thatrespective pawls 42 directly contact extension tabs 16 and grooves 38and such that protuberances 56 are aligned with the opposing lateralopenings of the rod receiving slot 18 of their respective assemblies 10.Each collar 22 is allowed to readily slide distally along tabs 16 byvirtue of the ratcheting effect created between pawls 42 and grooves 38.In particular, pawls 42 are allowed to pivot away from extension tabs 16as teeth 80 of pawls 42 slide over and around the ridges formed bygrooves 38 by virtue of the geometries of teeth 80, grooves 38 and theridges. In this manner, each collar 22 is moved distally along extensiontabs 16 until protuberances 56 meet resistance from rod 20.

Referring to FIG. 10, after the resistance posed by 20 is experienced,each of collars 22 must be manually pressed distally to overcome theproximally directed pressure of rod 20. This is accomplished byprogressively pressing each of collars 22 in succession toward distalend 11 until the proximally directed pressure exerted by rod 20 isperceived to be great enough to warrant the pressing of other of collars22, i.e., when the distally directed pressure is great enough to pullscrew 12 from the bone. When the manually applied distally directedpressure is discontinued for a particular collar 22, the collar isprevented from sliding proximally by the resulting proximally directedpressured exerted by rod 20 via the interaction of pawls 42 with grooves38, and more particularly, teeth 80 with the ridges formed betweenadjacent grooves 38. Teeth 80 are angled to lock within grooves 38 andprevent pivoting of pawls 42 about pivot point 44. This way, asillustrated in FIG. 11, the physician can move from assembly 10 toassembly 10 along a patient's spine and successively reduce rod 20within each rod seat 30 of assemblies 10. In the event there is a needto slide rod 20 proximally, pawls 42 can be disengaged from grooves 38by applying pressure to exterior end 48 of each pawl 42 thereby pivotinginterior end 52 away from extension tabs 16. This temporarily terminatesthe ratcheting interaction between collar 22 and tabs 16.

As depicted in FIG. 12, once rod 20 is fully seated within a rod seat,set screw 24 can be inserted between extension tabs 16, through rodreceiving slot 18 and opening 40 of collar 22 and screwed into threads32 of screw head 14 thereby locking rod 20 with screw head 14. Since therod receiving slot 18 a diameter that is greater than a diameter of setscrew 22, set screw 32 can be translated distally through slot 18without the need of screwing it onto threads. Thus, set screw 32 doesnot have to be rotated until it enters screw head 14 to engage threads32 therein. After rod 20 is locked in rod seat 30, extension tabs 16 canbe removed from screw head 14 at notches 34 by simply pressing tabslaterally.

As will be apparent to one skilled in the art, various modifications canbe made within the scope of the aforesaid description. Suchmodifications being within the ability of one skilled in the art form apart of the present invention and are embraced by the claims below.

It is claimed:
 1. A screw and rod fixation assembly comprising, a pairof opposing tabs having a proximal end and a distal end, the pair ofopposing tabs being coupled to one another about the distal end thereofby a screw head member, a rod receiving slot between the pair ofopposing tabs, and a sliding member configured for coupling to andbetween the pair of opposing tabs, wherein the sliding member isconfigured for allowing movement of the sliding member distally alongthe pair of opposing tabs when the sliding member is pushed along thepair of opposing tabs by a human hand exerting a force directed towardsthe screw head member and preventing movement of the sliding memberproximally along the pair of opposing tabs.
 2. The assembly according toclaim 1 wherein the screw head member is coupled with a pedicle screw.3. The assembly according to claim 1 further comprising a pawl extendingto and between a tab of the pair of opposing tabs and the slidingmember.
 4. The assembly according to claim 3 wherein the pawl isintegral with the sliding member and slidably coupled with the tab ofthe pair of opposing tabs.
 5. The assembly according claim 4 wherein thesliding member includes a release for selectively disengaging the pawlfrom a pawl receiving groove in the tab of the pair of opposing tabs. 6.The assembly according to claim 1 wherein the sliding member includes acollar having an opening for receiving the pair of opposing tabs and aset screw therethrough.
 7. The assembly according to claim 6 wherein thepair of opposing tabs are received within the opening and at least onepawl is extending between the collar and a tab of the pair of opposingtabs.
 8. The assembly according to claim 6 wherein the collar includesat least one protuberance extending from a lower edge thereof, the atleast one protuberance being aligned with the rod receiving slot.
 9. Theassembly according to claim 1 further comprising a rod positioned withinthe rod receiving slot and between the screw head member and the slidingmember, the sliding member pressing against the rod.
 10. The assemblyaccording to claim 9 further comprising a set screw threaded between thepair of opposing tabs and proximal to the rod, the set screw pressingthe rod against a rod seat within the screw head member.
 11. Theassembly according to claim 1 wherein the sliding member includes aflexible portion.
 12. A method for reduction of a rod into the screwhead member of claim 1 comprising, inserting the rod into the rodreceiving slot, coupling the sliding member to the pair of opposingtabs, and pushing the sliding member along the pair of opposing tabstoward the distal end thereof thereby directing the rod into a rod seatof the screw head member.
 13. An assembly comprising. extension tabsdetachably coupled to a screw head, a rod receiving slot formed betweenthe extension tabs, a collar configured for being pushed along theextension tabs toward the screw head with a hand exerting a forcedirected towards the screw head, and a ratchet mechanism configured forpreventing movement in a predetermined direction between the collar andthe extension tabs.
 14. The assembly according to claim 13 wherein theratchet mechanism includes at least one pawl.
 15. The assembly accordingto claim 14 wherein the at least one pawl is biased toward a centralaxis of the collar.
 16. The assembly according to claim 13 wherein theratchet mechanism is coupled to and between the collar and the extensiontabs.
 17. The assembly according to claim 13 wherein the ratchetmechanism includes a first portion that is integral with the collar anda second portion that is integral with the extension tabs.
 18. Theassembly according to claim 17 wherein the first portion is detachablycoupled with the second portion.
 19. The assembly according to claim 17wherein the first portion includes pawls and the second portion includespawl receiving grooves.
 20. The assembly according to claim 17 whereinthe first portion is movable relative to the collar and the secondportion is stationary relative to the extension tabs.